i) Field of the Invention
The present invention relates to the reprocessing of nuclear fuel, and more particularly to a continuous denitration apparatus for continuously extracting an oxide of nuclear fuel from a nitric acid solution thereof.
ii) Description of the Related Arts
In general, uranium nitrate and plutonium nitrate recovered in the reprocessing of nuclear fuel and a mixture of these substances are denitrated to convert into raw material powder by using a microwave heating direct denitration method. Two methods of reprocessing nuclear fuel using microwave heating are known: a batch processing method and a continuous processing method. In the batch method, a certain amount of solution of uranium nitrate or the like is poured into a container such as a so-called denitration bowl or tray and an evaporation-concentration, a denitration and a dying of the solution are executed in a microwave electric field. In the continuous method, the solution is successively processed in an evaporation-concentration zone, a denitration zone and a drying zone. In order to realize the latter method, a screw continuous denitration apparatus is developed, wherein a solution to be processed is conveyed in succession by a rotation of a screw so as to carry out a series processes in the above-described zones.
In this apparatus, as a notable difference from the batch processing, by continuously performing the evaporation-concentration, the denitration and the drying steps on a time base, mass processing can be carried out However, in order to stably execute this processing, the above-described steps must be controlled under certain conditions such as predetermined ranges of temperature, concentration and the like.
FIG. 3 illustrates a conventional continuous denitration apparatus. In FIG. 3, (A) is an elevational view, (B) is a side view and (C) is a top plan view. As shown in FIG. 3, a pair of long screws 12-1 and 12-2 are contained within a trough 11 having a long box form and are rotated in the directions by a drive motor (not shown), as shown by arrows in FIG. 3 (B). This trough 11 is isolated from a space formed by an upper oven 15 by a partition plate 13 composed of quartz glass, and the trough 11 is provided with a liquid feed inlet 14 for feeding a nitrate solution such as uranium nitrate of the nuclear fuel into the trough 11, an outlet 16 for discharging a nuclear fuel oxide produced as a result of a denitration processing, and a gas outlet 22 for discharging gases or the like produced within the trough 11.
The oven 15 is provided with four microwave incident inlets 21-1 to 21-4 on its top and a microwave with a predetermined frequency is incident to the oven 15 from the microwave incident inlets 21-1 to 21-4. In this apparatus, he space formed by the oven 15 is divided into three areas by microwave reflection boxes 18 and 19. That is, the first area is a concentration zone, in which the uranium nitrate solution is heated and concentrated by the microwave incident from the microwave incident inlet 21-1. The second area is a denitration zone where the uranium nitrate solution concentrated to a supersaturated condition in the concentration zone is denitrated by the microwave entered from the microwave incident inlet 21-2 to produce an oxide (for example, uranium trioxide) of the nuclear fuel. The third area is a drying zone, in which the oxide produced in the denitration zone is dried by the microwave irradiated through the microwave incident inlets 21-3 and 21-4 to obtain powder to be discharged from the outlet 16.
The trough 11 as a whole is installed at a minute inclination angle with respect to the horizontal level, and the uranium nitrate solution supplied from the liquid feed inlet 14 forms a liquid fraction part 24 extending to approximately the middle part of the denitration zone, as shown by an inclined line in FIG. 3 (A).
FIG. 4 conceptually shows the contents of the processing in such a screw continuous denitration apparatus. In FIG. 4, the horizontal axis represents the position in the screw rotation shaft direction and the vertical axis represents the temperature of the object to be processed in each position. As shown in FIG. 4, in the concentration zone, the uranium nitrate solution is heated to a boiling state and is sent to the denitration zone via the reaction process shown in the following formulas (1) to (4). EQU UO.sub.2 (NO.sub.3).sub.2.6H.sub.2 O.fwdarw.UO.sub.2 (NO.sub.3).sub.2.2H.sub.2 O+4H.sub.2 O (1) EQU UO.sub.2 (NO.sub.3).sub.2.2H.sub.2 O.fwdarw.UO.sub.2 (NO.sub.3).sub.2 +2H.sub.2 O (2) EQU UO.sub.2 (NO.sub.3).sub.2 +H.sub.2 O.fwdarw.UO.sub.2 (OH)NO.sub.3+HNO.sub.3( 3) EQU UO.sub.2 (OH)NO.sub.3 .fwdarw..beta.-UO.sub.3 +1/2H.sub.2 O+NO.sub.2 +1/40.sub.2 ( 4)
In the front end of the liquid fraction part 24 of the denitration zone, the uranium nitrate solution is in a supersaturated condition with an extremely high viscosity, and, while adhering to the lower part of the trough 11 and the blades of the screws 12-1 and 12-2 in the denitration zone, the high viscosity substance is denitrated to produce a denitrated product.
As described above, in order to quickly cause the denitration reaction, it is necessary to heat the solution to be processed to concentrate the solution to the supersaturated condition to reach a metal production concentration of the denitrated product such as UO.sub.3. However, in the aforementioned conventional denitration apparatus, due to wraparound effect of the microwave from the microwave incident inlets 21-1 and 21-2, in a considerably wide range 26 including a part of the concentration zone, as shown in FIG. 3 (A), the solution to be processed reaches the supersaturated condition and thus the high viscosity substance can be produced in the entire concentration zone. Such a solution in the supersaturated condition can readily be solidified by a slight temperature drop. It can be considered that this condition is attributable to a formation of .gamma.-UO.sub.3 in place of .beta.-UO.sub.3 in formula (4), and the deposited uranium having a sherbet form sticks to not only the blades of the screws but also the screw shafts, as shown in FIG. 5, and gradually grows. It is hard to exfoliate the stuck uranium from the screws and the stuck uranium can sometimes stop the rotation of the screws to render operation impossible.
In order to prevent such a supersaturated condition of the whole concentration part, it can be considered further heat the concentration part for keeping it highly soluble condition. Hence, it is sufficient to successively control the output of the microwave. However, in general, such a control is troublesome, and, when the output of the microwave is raised to excess, electric discharge tends to be caused in the oven or in corner parts of the microwave reflection boxes. Also, by carrying out such a control, electric field strength distribution in each zone becomes unstable. In particular, sufficient electric power required for the denitration zone is not supplied and the preparation of the denitrated product can not be stably performed. Further, when the electric field strength distribution becomes unstable, the denitrated product (UO.sub.3) sent to the drying zone is partially heated by applying the microwave to produce triuranium octoxide or uranyl uranate (U.sub.3 O.sub.8) and the triuranium octoxide (U.sub.3 O.sub.8) further absorbs the microwaves to grow, thereby obtaining an uneven mixture product (oxides of the nuclear fuel) containing UO.sub.3 and U.sub.3 O.sub.8.
On the other hand, another method is known, wherein, when the concentration of the whole concentration zone is raised to start the supersaturated condition forming, nitric acid is supplied from the liquid feed inlet 14 in order to dilute the solution. However, in this method, a drop of a processing amount is liable and it is difficult to produce a uniform product. Hence, this method is not advantageous.
As described above, in the conventional continuous denitration apparatus, in the evaporation-concentration process, the uranium in the solution reaches the supersaturated condition and is deposited to stop the rotation of the screws, and in the drying process, the produced uranium trioxide absorbs the microwaves to produce the triuranium octoxide. As a result, the uniform product (uranium trioxide) can not be obtained.